Thyroxine Enhances Breast Cancer Cell Survival and Proliferation via TRβ1-Dependent PI3K/AKT Signaling

Thyroid hormones (TH) influence tumor biology through both genomic and non-genomic mechanisms. Specifically, thyroxine (T4) activates signaling pathways linked to cancer progression through interactions with nuclear receptors, such as TRβ1, and membrane receptors, including integrin αvβ3. Nevertheless, the precise role of T4 in breast cancer cell behavior and its underlying molecular mechanisms remain incompletely understood. The effects of physiological concentrations of T4 (10^-9 M) on proliferation, cell viability, apoptotic signaling, and activation of intracellular pathways were evaluated in human mammary cell lines. Tumor cell lines (MCF-7 and MDA-MB-231) and the non-tumor mammary epithelial cell line MCF-10A were treated with T4 alone or in combination with the thyroid hormone receptor antagonist 1-850. Cell proliferation was measured using the MTT assay, and viability was determined by trypan blue exclusion. Protein expression and signaling pathways were analyzed by Western blot, including assessment of apoptotic markers (caspases, PARP, Bax, Bcl-2), PCNA, steroid hormone receptors, and signaling mediators such as PI3K, AKT, and ERK. Immunocytochemistry was used to evaluate TRβ1, integrin αvβ3, and Ki67 expression. T4 treatment increased proliferation and survival in hormone-sensitive tumor cells, accompanied by modulation of apoptosis-related proteins and activation of the PI3K/AKT pathway. The antagonist 1-850 selectively attenuated TRβ1-dependent effects, enabling distinction between genomic and integrin-mediated mechanisms. These effects were observed exclusively in hormone-sensitive tumor cells. These findings support a role for T4 in breast cancer progression and identify TH-related signaling pathways as potential therapeutic targets.